Architectural coatings and paints are commonly used on buildings, floors and furnishings to provide decorative effects as well as to protect surfaces from corrosion, oxidation and other damage due to physical stress and/or environmental deterioration.
In general, paints are composed of a vehicle, pigments or other colourant agents, and additives to modulate the physical characteristics of the paint, such as drying time, ease of application, viscosity, resistance to wear, etc. The vehicle may be an organic solvent or water-based, and consists of resin and solvent components. Alkyd paints are referred to as organic solvent-based as the alkyd resin is diluted with organic solvents such as hydrocarbons. Latex or emulsion paints (collectively “latex paint”) are polymers dispersed in a water phase and so are called “water-based”. Latex paints and coatings are popular for use in buildings, as the paints are non-flammable, and the paints and tools for applying such paints are easy to clean up, requiring only water and/or water-based cleaning products.
In the absence of any pigments, alkyd and latex paints are uncoloured, and generally appear translucent to the eye. Titanium dioxide pigment is added to produce white paints. Other pigments may be added to alkyd and latex paints and coatings, to provide colour (“tint”), as well as provide other decorative effects, for example, opacity; matte or glossy effect; metallic or pearlescent finishes. Pigments may also be added to modulate other physical characteristics of the resultant paint or coating, such as hardness, durability and water resistance.
Pigments may be composed of inorganic compounds such as iron oxides and other transition metal compounds. Pigments may also be composed of organic compounds such as azo compounds, quinacridones, and copper phthalocyanine complexes. Pure pigments are generally provided in solid, dry powder form composed of primary particles, aggregates (clusters of particles rigidly joined together) and agglomerates (loose collections of primary particles and aggregates). To make coloured paint, pigments are incorporated into the paint as powders during paint manufacture to form a stable, colloidal dispersion of the pigment within the vehicle, The pigment must be evenly dispersed throughout the paint to form a uniform, homogeneous composition.
Solid dry pigments can be difficult or impossible to disperse into the base paint without the aid of a surface-active agent (“surfactant”) that lowers interfacial tension at phase boundaries, such as liquid-air, or between two immiscible liquids (for example, water-organic solvent) (Bieleman, J. “Chapter 4: Surface-Active Agents” in Additives for Coatings, J. Bieleman, Ed., Wiley-VCH: Weinheim, 2000). Due to their insolubility in most solvents and difficulty to disperse in liquid media, dry pigments are typically formulated into a concentrated liquid or wet paste (a “colourant concentrate”) which may then be added by the end user to an uncoloured or white finished paint, called the “base paint”. In order to make a colourant, the dry pigment is “wetted” with a vehicle containing a surfactant, wherein the pigment-air interface is replaced with a pigment-liquid medium interface. The pigment agglomerates are broken down mechanically and also wetted by the surfactant. Other additives can be added to aid dispersion of the pigment in the colourant, and to improve other performance characteristics, such as extending shelf-life, and modulating the physical characteristics of the resultant tinted paint.
A paint colourant comprises one or more pigments, and a carrier or vehicle for the pigment(s). Thus, the dispersing vehicle is of primary importance to the performance of any resultant paint colourant. The most significant component of any dispersing vehicle is a compound which has dispersant activity. One or more dispersant compounds are required for wetting the particles and agglomerates of pigment, for the formation of a uniform dispersion of the pigment during in the composition, and stabilization of the dispersion once formed. The dispersant aids in the inhibition of flocculation when the colourant is incorporated into the base paint, and eliminates precipitation and/or sedimentation of the pigment out of the composition. In order to have a colourant concentrate that is compatible with both water-based and organic solvent-based paints, it is necessary to have a vehicle for carrying the pigments that is miscible in both water and organic solvents.
To maximize industrial and commercial applicability, as well as for the purposes of economy and ease of use by the endpoint consumer, it is desirable to have a “universal” paint colourant that may be used to tint both water-based and organic solvent-based paints. As with the paints to be tinted, universal paint colourants may also be organic solvent-based or water-based. However, it is difficult formulate an organic solvent-based paint colourant that is suitable for dispersion into a water-based paint. For the purposes of preparing a universal colourant, it is preferable to have a colourant that is water/aqueous-based, and add surfactants that allow incorporation of the colourant into both organic solvent-based and water-based paints. However, water-based colourant concentrates require the presence of a humectant to retain water and maintain fluidity. The presence of a humectant is particularly important for maintaining fluidity in colourants which are provided in concentrated form.
To ensure the compatibility of universal colourants in both water-based and solvent-based paints, colourants typically contain relatively high levels of surfactants and other additives. However, the presence of these surfactants and additives can disrupt the rheological network formed by the associative thickeners in water-based paints, resulting in a reduction in viscosity when such base paints are tinted. When preparing deep coloured paints, relatively large quantities of universal colourant must be added to the base paint to achieve the desired effect. However, these base paints may exhibit a large drop in viscosity upon addition and mixing of the universal colourant. This viscosity drop can result in a number of negative effects on the performance of the resultant tinted paint, such as poor sag control, syneresis (expulsion of fluid from the composition) and settling out of pigments if the tinted paint is stored for some time. Therefore, universal colourants must be formulated in such a way so as to avoid such a viscosity drop yet still allow maximum compatibility with a wide variety of base paints.
A relatively small series of different universal paint colourants may be added in various amounts and combinations to an untinted base paint, in order to provide a large number of different paint colours. Paints and coatings may be tinted with a colourant, such as a universal colourant, in large volumes and then sold as a coloured product. Paints may also be tinted “in-store” prior to sale to the end consumer. In-store tinting allows stores and businesses to offer many different colours of paints to customers while only needing to stock untinted base paints and a series of different universal colourants. In order to tint paints in-store, dispensing equipment is used to add specific volumes of the universal colourants to the base paints. Such equipment generally includes a storage tank or canister, and dispensing nozzles. As the equipment can be exposed to the atmosphere for long periods of time between use, any colourant left in the dispensing equipment may dry up and clog the dispensing nozzle, unless it has been formulated to include humectants that help to retain or attract water in the universal colourant composition and thus preserve fluidity. Also, any colourant held in storage before dispensing must be able to withstand long periods of storage, without any significant change, such as settling of pigments from the composition, and changes in viscosity and/or rheology. During such storage, there may be some exposure to air and fluctuations in temperature. Thus, universal paint colourants, particularly those used for in-store tinting, must have a number of performance characteristics to aid in delivery and dispensing, such as an acceptably long open-air dry time, and good long-term storage stability, while still maintaining its universal compatibility with various types of base paints. Furthermore, prior to use, the colourants must exhibit good “in-can” stability. Colourants may experience extremes in temperature during storage and transport to the point of use, and therefore, must have freeze-thaw stability.
Previously, low molecular weight glycols, in particular ethylene glycol and diethylene glycol, were most commonly used as the primary vehicle for preparing universal colourant concentrates. Ethylene glycol and diethylene glycol are highly effective wetting vehicles, preventing the re-agglomeration of pigment particles during storage. They are also highly effective humectants, allowing the colourant composition to retain water for relatively long periods of time. This in turns helps to retain fluidity of the colourant during storage and particularly during exposure to the atmosphere when residing within dispensing equipment. However, ethylene glycol and diethylene glycol are considered to be volatile organic compounds (VOCs). VOCs have vapour pressures at room temperature and normal atmospheric conditions that are considered significant, i.e. above 1.3 N/m2. If present in paints and coatings, VOCs contribute to both indoor and outdoor air pollution. In closed environments, VOCs can cause allergic reactions, respiratory irritations, and other negative health effects, particularly if the exposure is long-term. Upon application of the paint or coating, VOCs are often detectable as a strong odour that can linger for many days after application. In addition to being considered a VOC, ethylene glycol is toxic and suspected to be teratogenic.
In addition to a vehicle for the pigments in a colourant composition, one or more surfactants are usually present in a universal colourant to provide compatibility with the paints and coatings they are incorporated into. Alkyl phenol ethoxylates (APEs) are commonly used as the primary surfactant in conventional colourant compositions, particularly in universal colourants. Alkyl phenol ethoxylates commonly used by architectural coating manufacturers are octylphenol and nonylphenol ethoxolates with a degree of ethoxylation between 5 and 10 (Bieleman, J. PPCJ 1995, 3, 17). APEs, particularly nonylphenol ethoxylates, became popular surfactants in paint colourants due to their low cost combined with highly effective wetting and dispersing properties for pigments. However, the main degradation products of nonylphenol ethoxylates are nonylphenol and shorter chain APEs, alkylphenol diethoxylates and alkylphenol acetic acid. Nonylphenol tends to persist in the environment and is toxic to aquatic life. In addition, nonylphenol is an endocrine disruptor and has weak estrogenic activity in vivo.
Due to mounting concerns over the negative impact of VOCs on the environment and on human health, many jurisdictions around the world have imposed regulations on the VOC content in architectural coatings, while others are beginning to follow suit. As well, in light of growing awareness of the negative effect of both VOCs and APEs on human health and the environment, consumers are turning increasingly towards VOC-free or low-VOC paint compositions that are preferably also APE-free. Manufacturers have been able to formulate low/zero-VOC and APE-free untinted base paints without significantly altering performance characteristics compared to conventional base paints containing VOCs and APEs. Such low/zero-VOC and APE-free base paints are now readily available to consumers. However, it has proven difficult to formulate universal paint colourant compositions that are free of both VOCs and APEs, yet still have comparable compatibility and performance characteristics (for example, good storage stability, colour strength and tone and other physical properties) to their conventional counterparts, while remaining competitive in terms of price.
In order to reduce or eliminate VOCs in universal paint colourant compositions, it is necessary to replace the previously employed ethylene glycol and diethylene glycol with other wetting agents/dispersants and humectants which are less volatile. However, when less volatile alternatives such as propylene glycol and polypropylene glycols such as dipropylene glycol are used, they have been found to have a negative impact on the water-resistance of the resultant tinted paints. In addition, propylene glycol and dipropylene glycol have very strong odours which are unacceptable to consumers. In addition, emulsifiable oils, such as edible oils and those used in food and personal care/cosmetic manufacturing, have been attempted as alternatives (for example, oleic acid, castor oil and mineral oil) but have not matched the performance of ethylene glycol and diethylene glycol. Others have attempted to replace ethylene glycol and diethylene glycol with various polymers as dispersants (see, for example, U.S. Pat. No. 6,235,813; U.S. Pat. No. 6,312,513, and U.S. Pat. No. 7,442,724).
At the same time, manufacturers have sought to replace APEs with surfactants that are perceived to be non-toxic. Suitable alternatives to APEs, particularly nonylphenol ethoxylate, have been identified as secondary and branched alcohol ethoxylates, alkyl polymers and tallow derivatives (see for example, K. Holmberg, B. Jonsson, B. Kronberg and B. Lindman, Surfactants and Polymers in Aqueous Solution, 2nd ed.; Wiley & Sons: Chichester, 2003).
A number of VOC- and APE-free paint colourant compositions have been produced thus far (see for example, U.S. Pat. No. 5,814,144). However, none of the alternatives to VOCs and APEs combined have produced a universal paint colourant that is truly compatible with both organic solvent-based paints and water-based paints, and that appears to match the dispersant/humectant performance and low cost of ethylene glycol and diethylene glycol. Examples of VOC- and APE-free universal paint colourants that have been produced thus far often have inferior performance characteristics compared to their conventional counterparts. For example, such colourants may have unacceptably short open-air dry times, becoming too thick to be dispensed or drying out completely and clogging the dispensing equipment, particularly the dispensing nozzles. In such cases, stores and business offering in-store tinting of base paints have had to resort to retrofitting dispensing equipment with humidity chambers in order to maintain the viscosity of the colourant compositions. In other cases, VOC- and APE-free paint colourants that are presently available may have poor compatibility with both organic solvent-based and water-based paints, and may only be compatible with one type of paint. At the same time, such colourants may be significantly more expensive than conventional universal paint colourants.
Accordingly, there is a need for alternative universal paint colourants that are free of VOCs and APEs that have satisfactory performance characteristics, such as good storage stability, good freeze-thaw stability and ease of dispensing, while remaining competitive in terms of price when compared to conventional universal paint colourants.